1. Field of the Invention
The present invention relates to an adapter optical system to be attached to an image side of an image pickup lens used for a television camera, a digital still camera, a video camera, or the like in a detachable manner, the adapter optical system including an optical element that can be inserted in and removed from an optical path, and also relates to an image pickup apparatus including the adapter optical system.
2. Description of the Related Art
Conventionally, there are known many optical characteristic converting element (hereinafter, also referred to as simply “optical element”) to be disposed in the optical path so as to change optical characteristic. Embodiments of such optical element include a neutral density filter (ND filter) and a color temperature conversion filter. The optical characteristic converting element is not necessary in a situation where there is no need to change the optical characteristic, and hence the optical characteristic converting element must be removed from the optical path. When the optical element is inserted in and removed from the optical path, the thickness of the element may cause a change of an imaging position due to the insertion and removal. As a result, there is a practical problem that an image becomes blurred. To address this problem, there is proposed a technology for suppressing a change of the imaging position when the optical element is inserted and removed. For example, Japanese Patent Application Laid-Open No. S63-25612 discloses an invention of giving refractive power to an optical element to be inserted in order to correct a change of an imaging position when the optical element is inserted. When a parallel flat plate optical element is inserted between an image pickup lens and an image pickup surface, or in a lens system of the image pickup lens, the imaging position is shifted to the over side (namely, the opposite side to the object side). In Japanese Patent Application Laid-Open No. S63-25612, positive refractive power is given to the optical element to be inserted so that the movement of the imaging position is canceled. Japanese Patent No. 3861241 discloses that a change of the imaging position due to switching among a plurality of filters is canceled by moving the image pickup element in the optical axis direction.
However, in the conventional technologies described in Japanese Patent Application Laid-Open No. 563-25612 and Japanese Patent No. 3861241 which are described above, insertion and removal of the optical element causes a change in aberration. For instance, if refractive power is given to the optical element to be inserted so as to cancel the shift of the imaging position as described in Japanese Patent Application Laid-Open No. S63-25612, spherical aberration of the under side occurs when the optical element is inserted. FIG. 9 illustrates a schematic diagram of the spherical aberration when the shift of the imaging position is canceled as described in Japanese Patent Application Laid-Open No. S63-25612. In FIG. 9, a dashed dotted line 902 indicates aberration when the optical element is not inserted in the optical path, a broken line 901 indicates aberration when the optical element is inserted in the optical path, and a position 907 indicates a position of an image pickup surface. As illustrated in FIG. 9, quality of the obtained image is deteriorated, and in addition, spherical aberration is changed from the state indicated by the line 902 to the state indicated by the line 901. When a best focus position (903, 904) expressed as a position where a root-mean-square (RMS) deviation of a spot diameter of an axial beam in a spot diagram becomes smallest exceeds a focal depth (range between 905 and 906), an influence of the insertion and removal of the optical element to the image becomes conspicuous, and hence the obtained image is changed significantly.
Further, if a function of inserting and removing the optical characteristic converting element is added to an existing product, it is preferred that the insertion and removal of the optical characteristic converting element be performed readily and in short time as much as possible. However, with the above-mentioned technology disclosed in Japanese Patent Application Laid-Open No. S63-25612, a space is formed between the image pickup lens and an imaging surface, and hence it is necessary to redesign the image pickup lens itself. Also with the technology disclosed in Japanese Patent No. 3861241, it is necessary to additionally dispose a mechanism for driving the image pickup element.